This supporting "existingdir" function, while simple, supplied a
meaningful mnemonic to the "test for an existing directory", part of a
manufactured family of functions covering the available flags.

Also note, my cavalier attitude around the variable quoting. I never
did. This style dates from '83 at the dawn of the Korn Shell. Having
barely recovered from the 14-character limit on filenames, there were
never spaces in a file name (Bill Gates first showed up on a Jan '83
issue of Time mag.)

Mkdir is recursive: If the directory exists, do nothing, otherwise
create the parent directory, the ${1%/*} trims the last slash and
following characters, and then create the requested directory.

I may have used this for a decade after "-p" became available, since
I already had the functionality. Today's obsolescent function:

Mkdir () { mkdir -p "$1"; }

in case I bump into some old code.
ittoolbox.$

b.t.w. it was just this practice which led to my functional addiction.